In our previous study, we prepared tantalum oxide-based cathodes from oxy-tantalum phthalocyanines by heat treatment under low oxygen partial pressure for polymer electrolyte fuel cells. We revealed that the high onset potential for oxygen reduction reaction (ORR) indicated a low hydrogen peroxide formation rate due to the large extent of crystalline distortion of orthorhombic Ta2O5. However, the hydrogen peroxide formation rate was consistent with an apparent four-electron reaction rate because it included the effect of the hydrogen peroxide reduction reaction. Therefore, in this study, we calculated the individual rate constants of the ORR for tantalum oxide-based cathodes to obtain the four-electron reaction rate (four-electron reduction rate of O-2/two-electron reduction rate of O-2) and investigated the factors influencing the four-electron reaction rate. The values of the four-electron reaction rate for the tantalum oxide-based cathodes were larger than 1, indicating that the four-electron reaction mainly occurred on the oxide surface. A higher four-electron reaction rate was obtained in the case of severe crystalline distortion and shrinkage of orthorhombic Ta2O5 due to the large number of oxygen vacancies. (C) 2015 Elsevier Ltd. All rights reserved.